4cyl vs 3cyl

[rbargeron]

From the buyer's standpoint, there really isn't a compelling advantage of 4 vs 3 cylinders. From the viewpoint of a manufacturer, a full line of tractors will need a range of power output. It makes economic sense to produce engines with 2,3, 4 or more cylinders to cover that range. Tooling costs, common parts, etc. all help simplify the operation. Balance shafts and other tricks to reduce vibration are not needed for tractors - where owners are less concerned than a Lexus customer might be. The differences in actual thermal efficiency of the different configurations is not large. Over the years, tractor engines have had 1 to 6 pistons ranging from tea-cups to gallon paint cans - they all work.

 

[Monster5601]

I found another thread about this subject on this site from several years ago and after reading the posts, there is still no answer.

http://www.tractorbynet.com/forums/owning-operating/37654-4-cylinder-vs-3-cylinder.html

The math says that the 4 cylinder will produce smother torque. On a 4 stroke engine, the crank shaft has to turn two time for one power stroke or 720 degrees. Divide the number of cylinders in to 720 degrees to see how ofter, in degrees, you get a power stroke. Also, a power stoke last for 180 degrees of crank shaft rotation.

I wish I still worked in powertrain (GM) then I could go ask the design engineers this question.

 

[rbargeron]

........after reading the posts, there is still no answer............

If you ask the design engineers, they'll cite other factors too, like bore, stroke, flywheel weight, aspiration, and method of fueling, not to mention characterizing the load. Like anything else there's a lot to it.

 

[LD1]

Just for kicks.....off the top of my head.


2 main bearings
1 main cap
2 main cap bolts
2 rod bearings
1 rod cap
2 rod cap bolts
1 rod
1 wrist pin bushing
2 wrist pin keepers
1 wrist pin
1 piston
3 oil rings
2 compression rings
2 lifters
2 pushrods
2 rocker arms
2 rocker arm studs
2 rocker arm nuts
2 rocker arm pivot washers
2 valves
2 valve stem seals
4 valve locks
2 valve springs
2 valve spring retainers

45 fewer parts INSIDE the engine. This is not counting if the bolts have washers. And I could progagally break some things down farther, such as rod studs. This is just a rough Idea. Not counting outside the engine either, sucj as injectors and glowplugs.

 

[DavidAubke]

I had always thought that between two engines of the same overall size, the one with fewer cylinders would have more low-end torque due to the larger size of its cylinders.

I used to have two 550cc motorcycles. One a single-cylinder and the other a four-cylinder. The "thumper" felt like it really pulled at the low end but once the four-cylinder got its RPMs up, it felt faster.

 

[Egon]

The inherent smoothness of an engine depends on the number of cylinders and their orientation.


There are a few configurations that are inherently balanced. Others require additional mechanical items to balance them.:thumbsup:

There is no secret to it and it's well analyzed. Just try a Google search. It surely beats guessing!:thumbsup:

 

[Pushing_Tin]

Thanks for the replies, I was just wondering because I am looking at moving up in tractor size and all the JDs have 3 cyl and the Kubotas have 4 cylinders for about the same HP.

 

[PHILIP8N]

Thanks for the replies, I was just wondering because I am looking at moving up in tractor size and all the JDs have 3 cyl and the Kubotas have 4 cylinders for about the same HP.

I was looking at a JD3720 with a 3 cylinder before I bought my Kubota L3940. Both similar HP (Deere had more), 3 cylinder Deere smaller (92 cubes) engine than 4 cylinder Kubota (122 cubes). 3 cylinder had a turbo on it. Is this the same case as you??? The 3 cyl turbo probably has a better low end torque curve, but in this case I would rather have the larger 4 cylinder natually aspirated engine. FYI, all 4 cylinder engines in the Kubota Grand LXX40 line have dual balancer shafts. I once worked on a JD3029 3 cylinder in an air compressor and that was the worst enigne I have ever seen vibration wise. Philip.

 

[beppington]

I don't think the # of cylinders tells you much.

40 HP is 40 HP.

I think you need to go drive whichever 3 or 4 cylinder model you're curious about to know whether it vibrates more or less than any others, or more or less than you're expecting.

 

[Dr. Bob]

I have a B7800 ('03, purchased new) and I have never felt that it was too rough. Come on man, its a tractor not a Caddie! My 7800 has 400 hrs on it and has never failed to start nor had anything break (and it lives outside). I love this tractor. I agree with the others: get the tractor that can do the work you need (and then some just for "insurance").